Tilt rotor and tilt wing aircraft convert between a forward flight cruise mode and a hover mode by converting the orientation of their propellers or rotors and nacelles. FIG. 1 shows a typical prior art tiltrotor aircraft 100 comprising a wing 102 and fuselage 104 with a first rotor 120 and first nacelle 114 in aircraft cruise mode corresponding with a generally horizontal position of the nacelle 104. The aircraft is also equipped with a second rotor 130 on the opposite end of the wing 102. In a typical tilt rotor aircraft 100, the nacelle 104 is also capable of operation in a generally vertical position used in helicopter mode flight. The nacelle 104 tilt angle is usually effected using a tilt actuator and mechanism to convert from helicopter mode flight to aircraft cruise mode. The mechanism that enables this conversion plays an important role in the overall reliability and safety of the aircraft. The mechanism must be both reliable and robust. In the aircraft industry robustness often takes the form of fault tolerance. In this manner the device must not only be robust in the face of harsh operating conditions, it must also be arranged in such a manner that it will continue to operate at a functional level with one or more critical components non-functional. Weight is also a critical factor in the efficiency of all aircraft. Thus, the conversion mechanism must achieve safety and reliability at a minimum weight while being able to deal with the substantial rotor forces and moments encountered during conversion.
Several methods of converting an aircraft between a hover mode and a forward flight mode have been suggested, but all involve overcoming opposing aerodynamic and inertia forces using said actuator. One of the first operational tilt-rotor designs was the Bell™ XV-15, which achieves conversion of the nacelle through the use of a linear actuator on a three-bar mechanism. The XV-15 actuator is a linear actuator, and when extended, the angle between the nacelle and the wing increases. Dual redundant hydraulic systems are arranged on the common actuator to increase the reliability of the system. During forward flight in this and other fielded tilt rotor designs, the nacelle is in a horizontal position, with the rotor producing thrust in the horizontal direction. In this position, it is common practice to lock the nacelle when horizontal to reduce the load on the actuator.
The Bell™ V-22 was developed subsequently to the XV-15 demonstrator. This vehicle used a similar 3 bar mechanism to tilt the nacelle. As with the XV-15, the V-22 actuator generally comprises first and second hydraulic motors that act on a common axis. A third electric motor provides for triple redundancy. In this arrangement, a failure of the first motor will not stop the mechanism from functioning.
The most recent tilt rotor aircraft, the Bell™ 609 has a tilt mechanism (also known as a conversion actuator) that uses a generally similar linear actuator on a three bar linkage, where the wing and the nacelle comprise two of the three bars, and the actuator comprises the third. The Bell™ 609 also has a shaft interconnection between left tilt actuator and right tilt actuator, which allows one actuator to tilt both nacelles, but specifically disallows differential tilting. Aspects of the Bell™ 609 conversion actuator system are described in U.S. Pat. No. 6,220,545 to Fenny et al., and in U.S. Pat. No. 6,247,667 also to Fenny et al.
The Fenny references and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
FIG. 2 is a top view schematic view of the prior art Bell™ V-22 conversion system, showing the nacelle 204 coupled to the wing 202 by means of a tilt spindle 210. An actuator 214 connected to an actuator spindle 212 allows the nacelle 204 to rotate with respect to the wing 202 in order to convert between flight modes. An engine and gearbox 220 are coupled to a shaft 222 that is also a mast. The shaft 222 is coupled to the hub 228 by means of a gimbal joint 224 which allows rotation of the hub 228 with respect the shaft 222 in several directions and greatly reduces the transfer of bending moments from the hub 228 to the shaft 222 and nacelle 204. A blade 230 that can change pitch is coupled to the hub 228.
A small unmanned Bell™ tiltrotor, known as the EagleEye™, is also equipped with a conventional gimbaled rotor and a conversion actuator system. Aspects of the tilt mechanism are described in WIPO Publication Number 2006/041455. This conversion actuator uses a four bar linkage in the tilt mechanism to facilitate nacelle tilt and conversion between flight modes. Methods of conversion described are conventional; the actuator is powered to force the nacelle, mast, and rotor to tilt to a desired conversion angle, reacting any aerodynamic or inertial forces on the rotor and
In general, prior art tiltrotors are of the gimbaled type, for which the blades and hub can be oriented somewhat independently of the mast and nacelle. That is, their rotors are allowed to tilt about a point at the hub to nacelle or blade to hub interface, but their masts remain stationary with respect to the non-rotating aircraft in any given flight mode. This hinging means the rotors do not transmit large moments from the rotor to the aircraft structure. Instead, the rotor transmits only a thrust vector. As such, these tilting mechanisms do not have to overcome the large rotor moments that would be associated with a hingeless tilt-rotor. For gimbaled rotors, aircraft yaw control can be achieved with application of rotor cyclic alone, effectively reorienting the hubs (and thus thrust vectors) of two rotors on either end of a wing to affect a yaw moment and thus a yaw rotation of the aircraft.
Differential nacelle tilt as a control concept has been considered in the prior art. The US Air Force report “Design studies and model tests of the stowed tilt rotor concept” by Bernard L. Fry suggests that a combination of differential nacelle tilt and rotor blade pitch cyclic for yaw control could reduce the high cyclic forces produced if yaw control is obtained with cyclic only. In a very different application, for a fighter with tilting jet engines differential nacelle tilt to control yaw has also been suggested as documented in “German V/STOL fighter program” by Albert C. Piccirillo, AIAA Press, 1997.
Since the prior art tilting mechanisms are all simple nacelle rotating devices, mechanized by linear actuators they are unable to accommodate very large moments that might be generated. For example, if one needed a nacelle moment of say 450,000 ft lb and an actuator arm of 1.5 ft, then the unit force would be 300,000 lb. The prior art actuator mechanisms cannot practically produce such large forces. Moreover, such these large forces place undesirably large demands on the actuator, especially when actuator failures must be accommodated for vehicle fault tolerance. Thus, there is still a need for apparatus, systems and methods that rotate the nacelle of a tilt rotor aircraft, especially in the case of a hingeless rotor where the rotor can induce a large moment.